Various dynamic braking circuits have been used in the past for slowing and stopping equipment, such as elevators and conveyors, driven by electrical motors. Since it is desirable in many systems to produce a bidirectional movement of the equipment, direct current motors are often used to drive such equipment, in which the polarity of the electrical current provided to the motor can be simply reversed to correspondingly reverse the direction of movement of the piece of equipment.
Some of the existing dynamic braking circuits enter a braking mode by short-circuiting the motor's armature, thereby producing a back-emf which in turn creates a reverse current generating a magnetic field which opposes the motor's rotation. One such circuit is disclosed in Whited (U.S. Pat. No. 3,786,329), which uses "smoothing inductors" in series with the reverse current to "smooth" the reverse current. The smoothing inductors have the effect of limiting the reverse current, so that the switching SCR's (silicon controlled rectifiers) are not required to conduct the very large reverse current produced by directly shorting the motor armature, which could destroy the SCR's if such current were not limited. A braking circuit which uses a braking resistor is disclosed in Meissen et al. (U.S. Pat. No. 3,876,920) in which the reverse current is limited by such braking resistors. Meissen et al. also discloses a mode of braking known as "regenerative" braking, whereby the motor is short-circuited by SCR's while the armature current (which becomes a reverse current) is fed back into the power system under certain circumstances, thereby charging a battery or a DC power source. Another braking circuit which uses a braking resistor and also has a regenerative braking mode is disclosed in Pham (U.S. Pat. No. 4,386,299).
A major disadvantage of using braking resistors and/or braking inductors is that, since the reverse current becomes limited, the magnetic field produced by such reverse current also is limited. By limiting the magnetic field that opposes the rotational inertia of the motor, the braking circuit fails to stop the motion of the piece of equipment as quickly as possible. Even existing equipment which provides a "hard" stopping mode does not absorb the maximum reverse current without some type of external components (i.e., other than the SCR's) for switching that armature's current.
In many conveyor applications, it is desirable to have a "soft" start along with a "hard" stop. In other words, the conveyor will start to move using a limited acceleration mode (a "soft" start), then build up to full speed. When it is desired to stop the conveyor's movement, a "hard" stop would be as sudden as the dynamic braking circuit will allow. Such soft start/hard stop controllers are useful for unidirectional applications, however, the existing soft start/hard stop controllers are limited for bidirectional applications in that they have a "mirrored" acceleration/deceleration capability. For example, if the conveyor is started in a "soft" mode while moving to the right, and then stopped in a "hard" mode while still moving to the right, that same conveyor, as it initiates movement in the opposite "left" direction, must necessarily undergo a "hard" start as it starts to accelerate to the left. Such conveyor will then have a "soft" stop as it slows down while still moving to the left. In many conveyor applications, the most desirable mode of starting and stopping is to have a soft start and hard stop regardless of the direction of the equipment or conveyor. Existing dynamic braking circuits cannot provide these desirable operational features.
Many of the dynamic braking electrical controllers available at the present time also require various capacitors and inductors to provide some noise immunity to limit destructive "false" triggers of the load current-bearing SCR's. Such false triggering still can occur in existing dynamic braking controllers, and often leads to irreversible destruction of such load current-bearing SCR's even when the transient noise impulses are relatively short in time duration.